Lens array, an exposure head and an image forming apparatus

ABSTRACT

An exposure head, includes: a lens array that includes lenses that are arranged in a first direction and in a second direction orthogonal to the first direction; and a light emitting element substrate that is provided with light emitting elements that emit lights to be imaged by the lenses, wherein a relationship defined by a following formula: 1&lt;L 1 /L 2  is satisfied, where the symbol L 1  denotes a length of the lens in the first direction and the symbol L 2  denotes a length of the lens in the second direction.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 12/357,231,filed Jan. 21, 2009, now U.S. Pat. No. 7,848,023 and claims the benefitof priority under 35 USC 119 of Japanese Patent Applications No.2008-14497 filed on Jan. 25, 2008 and No. 2008-304814 filed on Nov. 28,2008, which are incorporated herein by reference.

BACKGROUND

1. Technical Field

This invention relates to a lens array for imaging lights from lightemitting elements using lenses, an exposure head using the lens arrayand an image forming apparatus using the exposure head.

2. Related Art

A line head in which a plurality of substantially circular lenses arealigned in a longitudinal direction, for example, disclosed in FIG. 2and the like of JP-A-6-278314 is known as such an exposure head. In thisline head, the plurality of lenses are aligned in the longitudinaldirection and the respective lenses image lights incident from the lightemitting elements. A latent image carrier such as photosensitive drum isexposed by the lights imaged by the respective lenses to form a latentimage.

SUMMARY

In order to deal with an exposure at a higher resolution, a lens arraycan be formed by two-dimensionally arranging a plurality of lenses. Inother words, in this lens array, a plurality of lens rows each made upof a plurality of lenses aligned in the longitudinal direction (firstdirection) are arranged in a width direction (second direction)orthogonal to or substantially orthogonal to the longitudinal direction.

In light of a good exposure, it is preferable that large quantities oflights are incident on the lenses. Accordingly, it is, for example,thought to enlarge the lenses. However, since the lenses in the aboverelated art are substantially circular, pitches between the lenses inthe width direction (second direction) increase if the lenses areenlarged. Thus, there has been a possibility of enlarging the line head.Such enlargement of the line head causes a problem of cost increase.

An advantage of some aspects of the invention is to provide technologyenabling the miniaturization of a line head (exposure head) whileenabling a good exposure at a high resolution.

According to a first aspect of the invention, there is provided anexposure head, comprising: a lens array that includes lenses that arearranged in a first direction and in a second direction orthogonal tothe first direction; and a light emitting element substrate that isprovided with light emitting elements that emit lights to be imaged bythe lenses, wherein a relationship defined by a following formula:1<L1/L2 is satisfied, where the symbol L1 denotes a length of the lensin the first direction and the symbol L2 denotes a length of the lens inthe second direction.

According to a second aspect of the invention, there is provided a lensarray, comprising: lenses that are arranged in a first direction and ina second direction orthogonal to the first direction, wherein arelationship defined by a following formula: 1<L1/L2 is satisfied, wherethe symbol L1 denotes a length of the lens in the first direction andthe symbol L2 denotes a length of the lens in the second direction.

According to a third aspect of the invention, there is provided an imageforming apparatus, comprising: an exposure head that includes a lensarray which has lenses that are arranged in a first direction and in asecond direction orthogonal to the first direction, and a light emittingelement substrate that is provided with light emitting elements thatemit lights to be imaged by the lenses; and a latent image carrier thatis exposed by the exposure head to form a latent image, wherein arelationship defined by a following formula: 1<L1/L2 is satisfied, wherethe symbol L1 denotes a length of the lens in the first direction andthe symbol L2 denotes a length of the lens in the second direction.

In these aspects of the invention (exposure head, lens array, imageforming apparatus) thus constructed, the length L1 of the first lens inthe first direction and the length L2 thereof in the second directionare set to satisfy the following formula: 1<L1/L2. In other words, thelength of the lens in the second direction is set to be shorter, whereasthe length thereof in the first direction is set to be longer.Accordingly, larger quantities of lights can be incident on the lensesin the first direction while pitches between the lenses in the seconddirection are suppressed. Therefore, the exposure head can beminiaturized while a good exposure at a high resolution is enabled.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawing. It is to beexpressly understood, however, that the drawing is for purpose ofillustration only and is not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are diagrams showing terminology used in thisspecification.

FIG. 3 is a diagram showing an embodiment of an image forming apparatusincluding a line head as an application subject of the invention.

FIG. 4 is a diagram showing the electrical construction of the imageforming apparatus of FIG. 3.

FIG. 5 is a perspective view schematically showing a line head accordingto the invention.

FIG. 6 is a partial sectional view along a width direction of the linehead shown in FIG. 5 in which the cross section is parallel to theoptical axis of the lens.

FIG. 7 is a diagram showing the configuration of the under surface ofthe head substrate.

FIG. 8 is a diagram showing the configuration of the light emittingelement group provided on the under surface of the head substrate.

FIG. 9 is a plan view of the lens array according to this embodiment.

FIG. 10 is a longitudinal sectional view of the lens arrays, the headsubstrate and the like showing a longitudinal cross section including anoptical axis of the lens formed in the lens array.

FIG. 11 is a perspective view showing spots formed by the line head.

FIG. 12 is a diagram showing a spot forming operation by the above linehead.

FIG. 13 is a partial sectional view showing a second embodiment of theinvention.

FIG. 14 is a partial plan view showing the structure of the diaphragmsaccording to the second embodiment.

FIG. 15 is a plan view showing other structure of the light emittingelement groups.

FIG. 16 is a view showing the structure of the under surface of the headsubstrate on which the plurality of light emitting element groups shownin FIG. 15 are arranged.

FIG. 17 is a diagram showing the construction of a lens array accordingto another embodiment of the invention.

FIG. 18 is a view showing an optical system according to the example andshowing a cross section taken along the main scanning direction.

FIG. 19 is a partial cross sectional view of the line head and thephotosensitive drum taken along the line A-A according to the example.

FIG. 20 is a table showing optical data according to this example.

FIG. 21 is a table showing the data of the optical systems which includethe middle lenses.

FIG. 22 is a drawing of definitional equations which define the X-Ypolynomial surfaces.

FIG. 23 is a table of the coefficients indicative of the surfaces S4 ofthe optical systems which include the middle lenses.

FIG. 24 is a table of the coefficients indicative of the surfaces S7 ofthe optical systems which include the middle lenses.

FIG. 25 is a table showing data of an optical system including upstreamlenses and downstream lenses.

FIG. 26 is a table showing coefficient values of the surfaces S4 of theoptical system including the upstream and downstream lenses.

FIG. 27 is a table showing coefficient values of the surfaces S7 of theoptical system including the upstream and downstream lenses.

FIG. 28 is a table showing another numerical example and corresponds toa case where the diameter of the photosensitive drum is 30 [mm].

FIG. 29 is a table showing still another numerical example andcorresponds to a case where the diameter of the photosensitive drum is45 [mm].

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Terms used in this specification are first described below (see “A.Description of Terms”). Following this description of terms, embodimentsof the invention (see “B-1. First Embodiment” and the like) aredescribed.

A. Description of Terms

FIGS. 1 and 2 are diagrams showing terminology used in thisspecification. Here, terminology used in this specification is organizedwith reference to FIGS. 1 and 2. In this specification, a conveyingdirection of a surface (image plane IP) of a photosensitive drum 21 isdefined to be a sub scanning direction SD and a direction orthogonal toor substantially orthogonal to the sub scanning direction SD is definedto be a main scanning direction MD. Further, a line head 29 is arrangedrelative to the surface (image plane IP) of the photosensitive drum 21such that its longitudinal direction LGD corresponds to the mainscanning direction MD and its width direction LTD corresponds to the subscanning direction SD.

Collections of a plurality of (eight in FIGS. 1 and 2) light emittingelements 2951 arranged on the head substrate 293 in one-to-onecorrespondence with the plurality of lenses LS of the lens array 299 aredefined to be light emitting element groups 295. In other words, in thehead substrate 293, the plurality of light emitting element groups 295including a plurality of light emitting elements 2951 are arranged inconformity with the plurality of lenses LS, respectively. Further,collections of a plurality of spots SP formed on the image plane IP bylight beams from the light emitting element groups 295 imaged on theimage plane IP by the lenses LS corresponding to the light emittingelement groups 295 are defined to be spot groups SG. In other words, aplurality of spot groups SG can be formed in one-to-one correspondencewith the plurality of light emitting element groups 295. In each spotgroup SG the most upstream spot in the main scanning direction MD andthe sub scanning direction SD is particularly defined to be a firstspot. The light emitting element 2951 corresponding to the first spot isparticularly defined to be a first light emitting element.

A spot group row SGR and a spot group column SGC are defined as shown inthe column “On Image Plane” of FIG. 2. Specifically, a plurality of spotgroups SG arranged in the main scanning direction MD are defined as thespot group row SGR. A plurality of spot group rows SGR are arranged atspecified spot group row pitches Psgr in the sub scanning direction SD.Further, a plurality of (three in FIG. 2) spot groups SG arranged atspot group row pitches Psgr in the sub scanning direction SD and at spotgroup pitches Psg in the main scanning direction MD are defined as thespot group column SGC. The spot group row pitch Psgr is a distance inthe sub scanning direction SD between the geometric centers of gravityof two spot group rows SGR adjacent in the sub scanning direction SD,and the spot group pitch Psg is a distance in the main scanningdirection MD between the geometric centers of gravity of two spot groupsSG adjacent in the main scanning direction MD.

Lens rows LSR and lens columns LSC are defined as shown in the column of“Lens Array” of FIG. 2. Specifically, a plurality of lenses LS alignedin the longitudinal direction LGD is defined to be the lens row LSR. Aplurality of lens rows LSR are arranged at specified lens row pitchesPlsr in the width direction LTD. Further, a plurality of (three in FIG.2) lenses LS arranged at the lens row pitches Plsr in the widthdirection LTD and at lens pitches Pls in the longitudinal direction LGDare defined to be the lens column LSC. It should be noted that the lensrow pitch Plsr is a distance in the width direction LTD between thegeometric centers of gravity of two lens rows LSR adjacent in the widthdirection LTD, and that the lens pitch Pls is a distance in thelongitudinal direction LGD between the geometric centers of gravity oftwo lenses LS adjacent in the longitudinal direction LGD.

Light emitting element group rows 295R and light emitting element groupcolumns 295C are defined as in the column “Head Substrate” of FIG. 2.Specifically, a plurality of light emitting element groups 295 alignedin the longitudinal direction LGD is defined to be the light emittingelement group row 295R. A plurality of light emitting element group rows295R are arranged at specified light emitting element group row pitchesPegr in the width direction LTD. Further, a plurality of (three in FIG.2) light emitting element groups 295 arranged at the light emittingelement group row pitches Pegr in the width direction LTD and at lightemitting element group pitches Peg in the longitudinal direction LGD aredefined to be the light emitting element group column 295C. It should benoted that the light emitting element group row pitch Pegr is a distancein the width direction LTD between the geometric centers of gravity oftwo light emitting element group rows 295R adjacent in the widthdirection LTD, and that the light emitting element group pitch Peg is adistance in the longitudinal direction LGD between the geometric centersof gravity of two light emitting element groups 295 adjacent in thelongitudinal direction LGD.

Light emitting element rows 2951R and light emitting element columns2951C are defined as in the column “Light Emitting Element Group” ofFIG. 2. Specifically, in each light emitting element group 295, aplurality of light emitting elements 2951 aligned in the longitudinaldirection LGD is defined to be the light emitting element row 2951R. Aplurality of light emitting element rows 2951R are arranged at specifiedlight emitting element row pitches Pelr in the width direction LTD.Further, a plurality of (two in FIG. 2) light emitting elements 2951arranged at the light emitting element row pitches Pelr in the widthdirection LTD and at light emitting element pitches Pel in thelongitudinal direction LGD are defined to be the light emitting elementcolumn 2951C. It should be noted that the light emitting element rowpitch Pelr is a distance in the width direction LTD between thegeometric centers of gravity of two light emitting element rows 2951Radjacent in the width direction LTD, and that the light emitting elementpitch Pel is a distance in the longitudinal direction LGD between thegeometric centers of gravity of two light emitting elements 2951adjacent in the longitudinal direction LGD.

Spot rows SPR and spot columns SPC are defined as shown in the column“Spot Group” of FIG. 2. Specifically, in each spot group SG; a pluralityof spots SP aligned in the longitudinal direction LGD is defined to bethe spot row SPR. A plurality of spot rows SPR are arranged at specifiedspot row pitches Pspr in the width direction LTD. Further, a pluralityof (two in FIG. 2) spots arranged at the spot row pitches Pspr in thewidth direction LTD and at spot pitches Psp in the longitudinaldirection LGD are defined to be the spot column SPC. It should be notedthat the spot row pitch Pspr is a distance in the sub scanning directionSD between the geometric centers of gravity of two spot rows SPRadjacent in the sub scanning direction SD, and that the spot pitch Pspis a distance in the main scanning direction MD between the geometriccenters of gravity of two spots SP adjacent in the main scanningdirection MD.

B-1. First Embodiment

FIG. 3 is a diagram showing an embodiment of an image forming apparatusincluding a line head as an application subject of the invention. FIG. 4is a diagram showing the electrical construction of the image formingapparatus of FIG. 3. This apparatus is an image forming apparatus thatcan selectively execute a color mode for forming a color image bysuperimposing four color toners of black (K), cyan (C), magenta (M) andyellow (Y) and a monochromatic mode for forming a monochromatic imageusing only black (K) toner. FIG. 3 is a diagram corresponding to theexecution of the color mode. In this image forming apparatus, when animage formation command is given from an external apparatus such as ahost computer to a main controller MC having a CPU and memories, themain controller MC feeds a control signal and the like to an enginecontroller EC and feeds video data VD corresponding to the imageformation command to a head controller HC. This head controller HCcontrols line heads 29 of the respective colors based on the video dataVD from the main controller MC, a vertical synchronization signal Vsyncfrom the engine controller EC and parameter values from the enginecontroller EC. In this way, an engine part EG performs a specified imageforming operation to form an image corresponding to the image formationcommand on a sheet such as a copy sheet, transfer sheet, form sheet ortransparent sheet for OHP.

An electrical component box 5 having a power supply circuit board, themain controller MC, the engine controller EC and the head controller HCbuilt therein is disposed in a housing main body 3 of the image formingapparatus. An image forming unit 7, a transfer belt unit 8 and a sheetfeeding unit 11 are also arranged in the housing main body 3. Asecondary transfer unit 12, a fixing unit 13 and a sheet guiding member15 are arranged at the right side in the housing main body 3 in FIG. 3.It should be noted that the sheet feeding unit 11 is detachablymountable into the housing main body 3. The sheet feeding unit 11 andthe transfer belt unit 8 are so constructed as to be detachable forrepair or exchange respectively.

The image forming unit 7 includes four image forming stations Y (foryellow), M (for magenta), C (for cyan) and K (for black) which form aplurality of images having different colors. Each of the image formingstations Y, M, C and K includes a cylindrical photosensitive drum 21having a surface of a specified length in a main scanning direction MD.Each of the image forming stations Y, M, C and K forms a toner image ofthe corresponding color on the surface of the photosensitive drum 21.The photosensitive drum is arranged so that the axial direction thereofis substantially parallel to the main scanning direction MD. Eachphotosensitive drum 21 is connected to its own driving motor and isdriven to rotate at a specified speed in a direction of arrow D21 inFIG. 3, whereby the surface of the photosensitive drum 21 is transportedin the sub scanning direction SD which is orthogonal to or substantiallyorthogonal to the main scanning direction MD. Further, a charger 23, theline head 29, a developer 25 and a photosensitive drum cleaner 27 arearranged in a rotating direction around each photosensitive drum 21. Acharging operation, a latent image forming operation and a tonerdeveloping operation are performed by these functional sections.Accordingly, a color image is formed by superimposing toner imagesformed by all the image forming stations Y, M, C and K on a transferbelt 81 of the transfer belt unit 8 at the time of executing the colormode, and a monochromatic image is formed using only a toner imageformed by the image forming station K at the time of executing themonochromatic mode. Meanwhile, since the respective image formingstations of the image forming unit 7 are identically constructed,reference characters are given to only some of the image formingstations while being not given to the other image forming stations inorder to facilitate the diagrammatic representation in FIG. 3.

The charger 23 includes a charging roller having the surface thereofmade of an elastic rubber. This charging roller is constructed to berotated by being held in contact with the surface of the photosensitivedrum 21 at a charging position. As the photosensitive drum 21 rotates,the charging roller is rotated at the same circumferential speed in adirection driven by the photosensitive drum 21. This charging roller isconnected to a charging bias generator (not shown) and charges thesurface of the photosensitive drum 21 at the charging position where thecharger 23 and the photosensitive drum 21 are in contact upon receivingthe supply of a charging bias from the charging bias generator.

The line head 29 is arranged relative to the photosensitive drum 21 sothat the longitudinal direction thereof corresponds to the main scanningdirection MD and the width direction thereof corresponds to the subscanning direction SD. Hence, the longitudinal direction of the linehead 29 is substantially parallel to the main scanning direction MD. Theline head 29 includes a plurality of light emitting elements arrayed inthe longitudinal direction and is positioned separated from thephotosensitive drum 21. Light beams are emitted from these lightemitting elements toward the surface of the photosensitive drum 21charged by the charger 23, thereby forming an electrostatic latent imageon this surface.

The developer 25 includes a developing roller 251 carrying toner on thesurface thereof. By a development bias applied to the developing roller251 from a development bias generator (not shown) electrically connectedto the developing roller 251, charged toner is transferred from thedeveloping roller 251 to the photosensitive drum 21 to develop thelatent image formed by the line head 29 at a development position wherethe developing roller 251 and the photosensitive drum 21 are in contact.

The toner image developed at the development position in this way isprimarily transferred to the transfer belt 81 at a primary transferposition TR1 to be described later where the transfer belt 81 and eachphotosensitive drum 21 are in contact after being transported in therotating direction D21 of the photosensitive drum 21.

Further, the photosensitive drum cleaner 27 is disposed in contact withthe surface of the photosensitive drum 21 downstream of the primarytransfer position TR1 and upstream of the charger 23 with respect to therotating direction D21 of the photosensitive drum 21. Thisphotosensitive drum cleaner 27 removes the toner remaining on thesurface of the photosensitive drum 21 to clean after the primarytransfer by being held in contact with the surface of the photosensitivedrum.

The transfer belt unit 8 includes a driving roller 82, a driven roller(blade facing roller) 83 arranged to the left of the driving roller 82in FIG. 3, and the transfer belt 81 mounted on these rollers. Thetransfer belt unit 8 also includes four primary transfer rollers 85Y,85M, 85C and 85K arranged to face in a one-to-one relationship with thephotosensitive drums 21 of the respective image forming stations Y, M, Cand K inside the transfer belt 81 when the photosensitive cartridges aremounted. These primary transfer rollers 85Y, 85M, 85C and 85K arerespectively electrically connected to a primary transfer bias generator(not shown). As described in detail later, at the time of executing thecolor mode, all the primary transfer rollers 85Y, 85M, 85C and 85K arepositioned on the sides of the image forming stations Y, M, C and K asshown in FIG. 3, whereby the transfer belt 81 is pressed into contactwith the photosensitive drums 21 of the image forming stations Y, M, Cand K to form the primary transfer positions TR1 between the respectivephotosensitive drums 21 and the transfer belt 81. By applying primarytransfer biases from the primary transfer bias generator to the primarytransfer rollers 85Y, 85M, 85C and 85K at suitable timings, the tonerimages formed on the surfaces of the respective photosensitive drums 21are transferred to the surface of the transfer belt 81 at thecorresponding primary transfer positions TR1 to form a color image.

On the other hand, out of the four primary transfer rollers 85Y, 85M,85C and 85K, the color primary transfer rollers 85Y, 85M, 85C areseparated from the facing image forming stations Y, M and C and only themonochromatic primary transfer roller 85K is brought into contact withthe image forming station K at the time of executing the monochromaticmode, whereby only the monochromatic image forming station K is broughtinto contact with the transfer belt 81. As a result, the primarytransfer position TR1 is formed only between the monochromatic primarytransfer roller 85K and the image forming station K. By applying aprimary transfer bias at a suitable timing from the primary transferbias generator to the monochromatic primary transfer roller 85K, thetoner image formed on the surface of the photosensitive drum 21 istransferred to the surface of the transfer belt 81 at the primarytransfer position TR1 to form a monochromatic image.

The transfer belt unit 8 further includes a downstream guide roller 86disposed downstream of the monochromatic primary transfer roller 85K andupstream of the driving roller 82. This downstream guide roller 86 is sodisposed as to come into contact with the transfer belt 81 on aninternal common tangent to the primary transfer roller 85K and thephotosensitive drum 21 at the primary transfer position TR1 formed bythe contact of the monochromatic primary transfer roller 85K with thephotosensitive drum 21 of the image forming station K.

The driving roller 82 drives to rotate the transfer belt 81 in thedirection of the arrow D81 and doubles as a backup roller for asecondary transfer roller 121. A rubber layer having a thickness ofabout 3 mm and a volume resistivity of 1000 kΩ·cm or lower is formed onthe circumferential surface of the driving roller 82 and is grounded viaa metal shaft, thereby serving as an electrical conductive path for asecondary transfer bias to be supplied from an unillustrated secondarytransfer bias generator via the secondary transfer roller 121. Byproviding the driving roller 82 with the rubber layer having highfriction and shock absorption, an impact caused upon the entrance of asheet into a contact part (secondary transfer position TR2) of thedriving roller 82 and the secondary transfer roller 121 is unlikely tobe transmitted to the transfer belt 81 and image deterioration can beprevented.

The sheet feeding unit 11 includes a sheet feeding section which has asheet cassette 77 capable of holding a stack of sheets, and a pickuproller 79 which feeds the sheets one by one from the sheet cassette 77.The sheet fed from the sheet feeding section by the pickup roller 79 isfed to the secondary transfer position TR2 along the sheet guidingmember 15 after having a sheet feed timing adjusted by a pair ofregistration rollers 80.

The secondary transfer roller 121 is provided freely to abut on and moveaway from the transfer belt 81, and is driven to abut on and move awayfrom the transfer belt 81 by a secondary transfer roller drivingmechanism (not shown). The fixing unit 13 includes a heating roller 131which is freely rotatable and has a heating element such as a halogenheater built therein, and a pressing section 132 which presses thisheating roller 131. The sheet having an image secondarily transferred tothe front side thereof is guided by the sheet guiding member 15 to a nipportion formed between the heating roller 131 and a pressure belt 1323of the pressing section 132, and the image is thermally fixed at aspecified temperature in this nip portion. The pressing section 132includes two rollers 1321 and 1322 and the pressure belt 1323 mounted onthese rollers. Out of the surface of the pressure belt 1323, a partstretched by the two rollers 1321 and 1322 is pressed against thecircumferential surface of the heating roller 131, thereby forming asufficiently wide nip portion between the heating roller 131 and thepressure belt 1323. The sheet having been subjected to the image fixing,operation in this way is transported to the discharge tray 4 provided onthe upper surface of the housing main body 3.

Further, a cleaner 71 is disposed facing the blade facing roller 83 inthis apparatus. The cleaner 71 includes a cleaner blade 711 and a wastetoner box 713. The cleaner blade 711 removes foreign matters such astoner remaining on the transfer belt after the secondary transfer andpaper powder by holding the leading end thereof in contact with theblade facing roller 83 via the transfer belt 81. Foreign matters thusremoved are collected into the waste toner box 713. Further, the cleanerblade 711 and the waste toner box 713 are constructed integral to theblade facing roller 83. Accordingly, if the blade facing roller 83 movesas described next, the cleaner blade 711 and the waste toner box 713move together with the blade facing roller 83.

FIG. 5 is a perspective view schematically showing a line head accordingto the invention, and FIG. 6 is a partial sectional view along a widthdirection of the line head shown in FIG. 5 in which the cross section isparallel to the optical axis of the lens. As described above, the linehead 29 is arranged relative to the photosensitive drum 21 such that thelongitudinal direction LGD thereof corresponds to the main scanningdirection MD and the width direction LTD thereof corresponds to the subscanning direction SD. The longitudinal direction LGD and the widthdirection LTD are orthogonal to or substantially orthogonal to eachother. As described later, in this line head 29, a plurality of lightemitting elements are formed on the head substrate 293 and therespective light emitting elements emit light beams toward the surfaceof the photosensitive drum 21. Accordingly, in this specification, adirection orthogonal to the longitudinal direction LGD and to the widthdirection LTD and propagating from the light emitting elements towardthe photosensitive drum surface is referred to as a light beampropagation direction Doa. This light beam propagation direction Doa isparallel to or substantially parallel to optical axes OA to be describedlater.

The line head 29 includes a case 291, and a positioning pin 2911 and ascrew insertion hole 2912 are provided at each of the opposite ends ofsuch a case 291 in the longitudinal direction LGD. The line head 29 ispositioned relative to the photosensitive drum 21 by fitting suchpositioning pins 2911 into positioning holes (not shown) perforated in aphotosensitive drum cover (not shown) covering the photosensitive drum21 and positioned relative to the photosensitive drum 21. Further, theline head 29 is positioned and fixed relative to the photosensitive drum21 by screwing fixing screws into screw holes (not shown) of thephotosensitive drum cover via the screw insertion holes 2912 to befixed.

The head substrate 293, a light shielding member 297 and two lens arrays299 (299A, 299B) are arranged in the case 291. The inner side of thecase 291 is held in contact with a top surface 293-h of the headsubstrate 293, whereas an under lid 2913 is held in contact with anunder surface 293-t of the head substrate 293. This under lid 2913 ispressed against the inner side of the case 291 via the head substrate293 by fixing devices 2914. In other words, the fixing devices 2914 haveelastic forces for pressing the under lid 2913 toward the inner side(upper side in FIG. 6) of the case 291 and the interior of the case 291is light-tightly sealed (in other words, so that light does not leakfrom the interior of the case 291 and light does not enter the case 291from the outside) by the under lid being pressed by such elastic forces.The fixing devices 2914 are provided at a plurality of positions spacedapart in the longitudinal direction LGD of the case 291.

The light emitting element groups 295 formed by grouping a plurality oflight emitting elements are provided on the under surface 293-t of thehead substrate 293. The head substrate 293 is made of a lighttransmissive material such as glass, and light beams emitted from therespective light emitting elements of the light emitting element groups295 can transmit from the under surface 293-t of the head substrate 293to the top surface 293-h thereof. These light emitting elements arebottom emission-type organic EL (electro-luminescence) devices and arecovered by a sealing member 294. The detailed arrangement of the lightemitting elements on the under surface 293-t of the head substrate 293is as follows.

FIG. 7 is a diagram showing the configuration of the under surface ofthe head substrate and corresponds to a case where the under surface isseen from the top surface of the head substrate. FIG. 8 is a diagramshowing the configuration of the light emitting element group providedon the under surface of the head substrate. As shown in FIG. 7, thelight emitting element group 295 is formed by grouping eight lightemitting elements 2951. In each light emitting element group 295, eightlight emitting elements 2951 are arranged as follows. Specifically, asshown in FIG. 8, in the light emitting element group 295, four lightemitting elements 2951 are aligned in the longitudinal direction LGD toform a light emitting element row 2951R and two light emitting elementrows 2951R are arranged at a light emitting element row pitch Pelr inthe width direction LTD. The respective light emitting element rows2951R are displaced from each other in the longitudinal direction LGD bya light emitting element pitch Pel, so that the positions of therespective light emitting elements 2951 in the longitudinal directionLGD differ from each other. The light emitting element group 295 thusconfigured has a longitudinal width Wegg in the longitudinal directionLGD and a widthwise width Wegt in the width direction LTD, wherein thelongitudinal width Wegg is larger than the widthwise width Wegt.

A plurality of light emitting element groups 295 thus configured arearranged on the under surface 293-t of the head substrate 293.Specifically, three light emitting element groups 295 are arranged atpositions mutually different in the width direction LTD to form a lightemitting element group column 295C, and a plurality of light emittingelement group columns 295C are arranged in the longitudinal directionLGD. In each light emitting element group column 295C, three lightemitting element groups 295 are displaced from each other by the lightemitting element group pitch Peg in the longitudinal direction LGD, withthe result that positions PTE of the respective light emitting elementgroups 295 in the longitudinal direction LGD differ from each other. Inother words, on the under surface 2934 of the head substrate 293, aplurality of light emitting element groups 295 are aligned in thelongitudinal direction LGD to form a light emitting element group row295R, and three light emitting element group rows 295R are arranged atthe light emitting element group row pitches. Peg in the width directionLTD. Further, the respective light emitting element group rows 295R aredisplaced from each other by the light emitting element group pitch Pegin the longitudinal direction LGD, with the result that the positionsPTE of the respective light emitting element groups 295 in thelongitudinal direction LGD differ from each other. Thus, in thisembodiment, a plurality of light emitting element groups 295 aretwo-dimensionally arranged on the head substrate 293. In FIG. 7, thepositions of the light emitting element groups 295 are represented bythe center of gravity positions of the light emitting element groups295, and the positions PTE of the light emitting element groups 295 inthe longitudinal direction LGD are indicated by feet of perpendicularsto an axis of the longitudinal direction LGD from the positions of thelight emitting element groups 295.

The respective light emitting elements 2951 formed on the head substrate293 in this way emit light beams having an equal wavelength upon beingdriven, for example, by a TFT (thin film transistor) circuit or thelike. The light emitting surfaces of the light emitting elements 2951are so-called perfectly diffusing surface illuminants and the lightbeams emitted from the light emitting surfaces comply with Lambert'scosine law.

Referring back to FIGS. 5 and 6, description continues. The lightshielding member 297 is arranged in contact with the top surface 293-hof the head substrate 293. The light shielding member 297 is providedwith light guide holes 2971 for the respective plurality of lightemitting element groups 295. In other words, a plurality of light guideholes 2971 are formed in a one-to-one correspondence with the pluralityof light emitting element groups 295. The light guide holes 2971 areformed as holes penetrating the light shielding member 297 in the lightbeam propagation direction Doa. Further, two lens arrays 299 arearranged side by side in the light beam propagation direction Doa abovethe light shielding member 297 (at a side opposite to the head substrate293).

As described above, the light shielding member 297 provided with thelight guide holes 2971 for the respective light emitting element groups295 is arranged between the light emitting element groups 295 and thelens arrays 299 in the light beam propagation direction Doa.Accordingly, light beams emitted from the light emitting element groups295 propagate toward the lens arrays 299 through the light guide holes2971 corresponding to the light emitting element groups 295. Converselyspeaking, out of the light beams emitted from the light emitting elementgroups 295, those propagating toward other than the light guide holes2971 corresponding to the light emitting element groups 295 are shieldedby the light shielding member 297. In this way, all the lights emittedfrom one light emitting element group 295 propagate toward the lensarrays 299 via the same light guide hole 2971 and the mutualinterference of the light beams emitted from different light emittingelement groups 295 is prevented by the light shielding member 297.

FIG. 9 is a plan view of the lens array according to this embodiment andcorresponds to a case where the lens array is seen from an image planeside (in the light beam propagation direction Doa). The respectivelenses LS in FIG. 9 are formed on an under surface 2991-t of a lensarray substrate 2991 and the construction of this lens array substrateunder surface 29914 is shown in FIG. 9. Although the light emittingelement groups 295 are shown in FIG. 9, this is to show a correspondencerelationship of the light emitting element groups 295 and the lenses LSand the light emitting element groups 295 are not formed on the lensarray substrate under surface 2991-t. As shown in FIG. 9, one lens LS isprovided for each light emitting element group 295 in the lens array299. Specifically, in the lens array 299, three lenses LS are arrangedat different positions in the width direction LTD to form a lens columnLSC, and a plurality of lens columns LSC are arranged in thelongitudinal direction LGD. In each lens column LSC, three lenses LS aredisplaced from each other by the lens pitch Pls, with the result thatpositions PTL of the respective lenses LS in the longitudinal directionLGD differ from each other. In other words, in the lens array 299, aplurality of lenses LS are aligned in the longitudinal direction LGD toform a lens row LSR, and three lens rows LSR are arranged at the lensrow pitch Plsr in the width direction LTD. The respective lens rows LSRare displaced from each other by the lens pitch Pls in the longitudinaldirection LGD, and the positions PTL of the respective lenses LS in thelongitudinal direction LGD differ from each other. In this way, theplurality of lenses LS are two-dimensionally arranged in the lens array299. In FIG. 9, the positions of the lenses LS are represented by thetops of the lenses LS (that is, points where sag is maximum) and thepositions PTL of the lenses LS in the longitudinal direction LGD arerepresented by feet of perpendiculars to the axis in the longitudinaldirection LGD from the tops of the lenses LS.

As shown in FIG. 9, the most upstream lens row LSR in the widthdirection LTD is made up of upstream lenses LS-u. The outer periphery ofeach upstream lens LS-u includes an arcuate portion CAP-u convex towardthe upstream side in the width direction LTD and a straight portionLNP-u extending in the longitudinal direction LGD and is substantiallyfan-shaped. When the length of the upstream lenses LS-u in thelongitudinal direction LGD is an upstream lens longitudinal directionlength L1-u (lens longitudinal direction length L1) and the length ofthe upstream lenses LS-u in the width direction LTD is an upstream lenswidth direction length L2-u (lens width direction length L2), theupstream lenses LS-u are formed so that the following formula:L1-u>L2-uis satisfied. Further, the shape of the light guide holes 2971 formedcorresponding to the upstream lenses LS-u is also substantiallyfan-shaped (FIG. 5).

The middle lens row LSR in the width direction LTD is made up of middlelenses LS-m. The outer periphery of each middle lens LS-m includesarcuate portions CAP-m located at the opposite ends in the longitudinaldirection LGD and convex toward outer sides and straight portions LNP-mlocated at the opposite ends in the width direction LTD and extending inthe longitudinal direction LGD and is shaped to be substantially flat inthe longitudinal direction LGD. When the length of the middle lensesLS-m in the longitudinal direction LGD is a middle lens longitudinaldirection length L1-m (lens longitudinal direction length L1) and thelength of the middle lenses LS-m in the width direction LTD is a middlelens width direction length L2-m (lens width direction length L2), themiddle lenses LS-m are formed so that the following formula:L1-m>L2-mis satisfied. Further, the shape of the light guide holes 2971 formedcorresponding to the middle lenses LS-m is also flat (FIG. 5).

The most downstream lens row LSR in the width direction LTD is made upof downstream lenses LS-d. The outer periphery of each downstream lensLS-d includes an arcuate portion CAP-d convex toward the downstream sidein the width direction LTD and a straight portion LNP-d extending in thelongitudinal direction LGD and is substantially reversed fan-shaped.When the length of the downstream lenses LS-d in the longitudinaldirection LGD is a downstream lens longitudinal direction length L1-d(lens longitudinal direction length L1) and the length of the downstreamlenses LS-d in the width direction LTD is a downstream lens widthdirection length L2-d (lens width direction length L2), the downstreamlenses LS-d are formed so that the following formula:L1-d>L2-dis satisfied. Further, the shape of the light guide holes 2971 formedcorresponding to the downstream lenses LS-d is also substantiallyreversed fan-shaped (FIG. 5).

FIG. 10 is a longitudinal sectional view of the lens arrays, the headsubstrate and the like showing a longitudinal cross section including anoptical axis of the lens LS formed in the lens array. The lens array 299includes the light transmissive lens array substrate 2991 long in thelongitudinal direction LGD. In this embodiment, this lens arraysubstrate 2991 is made of a glass having a relatively small linearexpansion coefficient. Out of a top surface 2991-h and the under surface2991-t of the lens array substrate 2991, the lenses LS are formed on thetop surface 2991-h of the lens array substrate 2991. This lens array 299is formed by a method disclosed in JP-A-2005-276849 for example.Specifically, a mold formed with recesses in conformity with the shapeof the lenses LS is held in contact with a glass substrate as a lenssubstrate 2991. A clearance between the mold and the light transmissivesubstrate is filled with a light curing resin. When light is irradiatedto this light curing resin, the light curing resin is cured and thelenses LS are formed on the light transmissive substrate. After thelenses are formed by solidifying the light curing resin, the mold isreleased.

As described above, in this embodiment, the lens array 299 is made up ofthe lens array substrate 2991 and the lenses LS. Accordingly, a degreeof freedom in the construction of the lens array 299 is improved, forexample, by enabling the selection of different base materials for thelens array substrate 2991 and the lenses LS. Thus, the lens array 299can be appropriately designed depending on specification required forthe line head 29 and a good exposure by the line head 29 can be easilyrealized. Further, in this embodiment, the lenses LS are made of thelight curing resin that can be quickly cured upon light irradiation.Accordingly, the lenses LS can be easily formed, wherefore the cost ofthe lens array 299 can be reduced by simplifying the production processof the lens array 299. Furthermore, since the lens array substrate 2991is made of glass having a small linear expansion coefficient, a goodexposure can be realized independently of temperature by suppressing thedeformation of the lens array 299 caused by a temperature change.

In this line head 29, two lens arrays 299 (299A, 299B) having such aconfiguration are arranged side by side in the light beam propagationdirection Doa. These two lens arrays 299A, 29913 are opposed to eachother with a pedestal 296 located therebetween, and this pedestal 296fulfills a function of specifying the spacing between the lens arrays299A, 299B. Thus, in this embodiment, two lenses LS1, LS2 aligned in thelight propagation direction Doa are arranged for each light emittingelement group 295 (FIGS. 5, 6 and 10). An optical axis OA (chaindouble-dashed line in FIG. 10) passing the centers of the first andsecond lenses LS1, LS2 corresponding to the same light emitting elementgroup 295 is orthogonal to or substantially orthogonal to the undersurface 293-t of the head substrate 293. Here, the lens LS of the linehead 299A upstream in the light beam propagation direction Doa is thefirst lens LS1, and that of the line head 299B downstream in the lightbeam propagation direction Doa is the second lens LS2. In thisembodiment, since a plurality of lens arrays 299 are arranged side byside in the light beam propagation direction Doa, a degree of freedom inoptical design can be increased.

As described above, the line head 29 is provided with an imaging opticalsystem including the first and the second lenses LS1, LS2. Accordingly,light beams emitted from the light emitting element groups 295 areimaged by the first and the second lenses. LS1, LS2 to form spots SP onthe photosensitive drum surface (image plane). On the other hand, thephotosensitive drum surface is charged by the charger 23 prior to spotformation as described above. Thus, areas where the spots SP are formedare neutralized to form spot latent images Lsp. The spot latent imagesLsp thus formed are conveyed toward a downstream side in the subscanning direction SD while being carried on the photosensitive drumsurface. As described next, the spots SP are formed at timings inconformity with the movement of the photosensitive drum surface to forma plurality of spot latent images Lsp aligned in the main scanningdirection MD.

FIG. 11 is a perspective view showing spots formed by the line head. Thelens array 299 is not shown in FIG. 11. As shown in FIG. 11, therespective light emitting element groups 295 can form the spot groups SGin exposure regions ER mutually different in the main scanning directionMD. Here, the spot group SG is a set of a plurality of spots SP formedby the simultaneous light emissions of all the light emitting elements2951 of the light emitting element group 295. As shown in FIG. 11, threelight emitting element groups 295 capable of forming the spot groups SGin the exposure regions ER consecutive in the main scanning direction MDare displaced from each other in the width direction LTD. In otherwords, three light emitting element groups 295_1, 295_2 and 295_3capable of forming spot groups SG_1, SG_2 and SG_3, for example, inexposure regions ER_1, ER_2 and ER_3 consecutive in the main scanningdirection MD are displaced from each other in the width direction LTD.These three light emitting element groups 295 constitute the lightemitting element group column 295C, and a plurality of light emittingelement group columns 295C are arranged in the longitudinal directionLGD. As a result, three light emitting element group rows 295R_A, 295R_Band 295R_C are arranged in the width direction LTD and the respectivelight emitting element group rows 295R_A, etc. form the spot groups SGat positions mutually different in the sub scanning direction SD asalready described in the description of FIG. 7.

Specifically, in this line head 29, the plurality of light emittingelement groups 295 (for example, light emitting element groups 295_1,295_2, 295_3) are arranged at positions mutually different in the widthdirection LTD. The respective light emitting element groups 295 arrangedat the positions mutually different in the width direction LTD form spotgroups SG (for example, spot groups SG_1, SG_2, SG_3) at positionsmutually different in the sub scanning direction SD.

In other words, in this line head 29, the plurality of light emittingelements 2951 are arranged at positions mutually different in the widthdirection LTD. For example, the light emitting elements 2951 belongingto the light emitting element group 295_1 and those belonging to thelight emitting element group 295_2 are arranged at positions mutuallydifferent in the width direction LTD. The respective light emittingelements 2951 arranged at the positions mutually different in the widthdirection LTD form spots SP at positions mutually different in the subscanning direction SD. For example, spots SP belonging to the spot groupSG_1 and those belonging to the spot group SG_2 are formed at positionsmutually different in the sub scanning direction SD.

In this way, the formation positions of the spots SP in the sub scanningdirection SD differ depending on the light emitting elements 2951.Accordingly, in order to form a plurality of spot latent images Lsp sideby side in the main scanning direction MD (that is, in order to form aplurality of spot latent images Lsp side by side at the same position inthe sub scanning direction SD), differences in such spot formationpositions need to be considered. Thus, in this line head 29, therespective light emitting elements 2951 are driven at timings inconformity with the movement of the photosensitive drum surface.

FIG. 12 is a diagram showing a spot forming operation by the above linehead. The spot forming operation by the line head is described withreference to FIGS. 7, 11 and 12. Briefly, the photosensitive drumsurface (latent image carrier surface) is moved in the sub scanningdirection SD and the head control module 54 (FIG. 4) drives the lightemitting elements 2951 for light emission at timings in conformity withthe movement of the photosensitive drum surface, whereby a plurality ofspot latent images Lsp arranged in the main scanning direction MD areformed.

First of all, out of the light emitting element rows 2951R (FIG. 11)belonging to the most upstream light emitting element groups 295_1,295_4, and the like in the width direction LTD, the light emittingelement rows 2951R downstream in the width direction LTD are driven forlight emission. A plurality of light beams emitted by such a lightemitting operation are imaged by the lenses LS to form spots SP on thephotosensitive drum surface. The lenses LS have an inversioncharacteristic, so that the light beams from the light emitting elements2951 are imaged in an inverted manner. In this way, spot latent imagesLsp are formed at hatched positions of a “First Operation” of FIG. 12.In FIG. 12, white circles represent spots that are not formed yet, butplanned to be formed later. In FIG. 12, spots labeled by referencenumerals 295_1 to 295_4 are those to be formed by the light emittingelement groups 295 corresponding to the respective attached referencenumerals.

Subsequently, out of the light emitting element rows 2951R belonging tothe most upstream light emitting element groups 295_1, 295_4, and thelike in the width direction, the light emitting element rows 2951Rupstream in the width direction LTD are driven for light emission. Aplurality of light beams emitted by such a light emitting operation areimaged by the lenses LS to form spots SP on the photosensitive drumsurface. In this way, spot latent images Lsp are formed at hatchedpositions of a “Second Operation” of FIG. 12. Here, the light emittingelement rows 2951R are successively driven for light emission from theone downstream in the width direction LTD in order to deal with theinversion characteristic of the lenses LS.

Subsequently, out of the light emitting element rows 2951R belonging tothe second most upstream light emitting element groups 295_2 and thelike in the width direction, the light emitting element rows 2951Rdownstream in the width direction LTD are driven for light emission. Aplurality of light beams emitted by such a light emitting operation areimaged by the lenses LS to form spots SP on the photosensitive drumsurface. In this way, spot latent images Lsp are formed at hatchedpositions of a “Third Operation” of FIG. 12.

Subsequently, out of the light emitting element rows 2951R belonging tothe second most upstream light emitting element groups 295_2 and thelike in the width direction, the light emitting element rows 2951Rupstream in the width direction LTD are driven for light emission. Aplurality of light beams emitted by such a light emitting operation areimaged by the lenses LS to form spots SP on the photosensitive drumsurface. In this way, spot latent images Lsp are formed at hatchedpositions of a “Fourth Operation” of FIG. 12.

Subsequently, out of the light emitting element rows 2951R belonging tothe third most upstream light emitting element groups 295_3 and the likein the width direction, the light emitting element rows 2951R downstreamin the width direction LTD are driven for light emission. A plurality oflight beams emitted by such a light emitting operation are imaged by thelenses LS to form spots SP on the photosensitive drum surface. In thisway, spot latent images Lsp are formed at hatched positions of a “FifthOperation” of FIG. 12.

Finally, out of the light emitting element rows 2951R belonging to thethird most upstream light emitting element groups 295_3 and the like inthe width direction, the light emitting element rows 2951R upstream inthe width direction LTD are driven for light emission. A plurality oflight beams emitted by such a light emitting operation are imaged by thelenses LS to form spots SP on the photosensitive drum surface. In thisway, spot latent images Lsp are formed at hatched positions of a “SixthOperation” of FIG. 12. By performing the first to sixth light emittingoperations in this way, a plurality of spots SP are successively formedfrom the upstream ones in the sub scanning direction SD to form aplurality of spot latent images Lsp aligned in the main scanningdirection MD.

As described above, in this embodiment, the lens LS is formed to satisfythe following formula:L1>L2where L1 (lens longitudinal direction length) denotes the length of eachlens LS in the longitudinal direction and L2 (lens width directionlength) thereof in the width direction. In other words, the length ofeach lens in the width direction LTD is set to be shorter, whereas thelength thereof in the longitudinal direction LGD is set to be longer.Accordingly, larger quantities of lights can be incident on the lensesin the longitudinal direction LGD while pitches (corresponding to thelens row pitch Plsr) between the lenses in the width direction. LTD aresuppressed. Therefore, in this embodiment, the line head 29 can beminiaturized while a good exposure at a high resolution is enabled.

As described with reference to FIGS. 11 and 12, the surface of thephotosensitive drum (surface of the latent image carrier) is moved inthe sub scanning direction SD (second direction) in this embodiment. Thelight emitting elements 2951 of the line head 29 are driven for lightemission at timings in conformity with the movement of the surface ofthe photosensitive drum 21 to expose the surface of the photosensitivedrum 21. In other words, light beams are successively imaged by the lensrows LSR from the most upstream one in the width direction LTD (subscanning direction SD) to form spot latent images Lsp. There are caseswhere the circumferential speed of the photosensitive drum 21 varies dueto the eccentricity of the photosensitive drum 21 or the like.Specifically, the above circumferential speed may vary until the nextlens row forms spot latent images Lsp after a certain lens row LSR formsspot latent images Lsp. As a result, there were cases where theformation positions of the spot latent images Lsp were displaced in thewidth direction LTD between the lens rows LSR and the spot latent imagesLsp could not be aligned in the main scanning direction MD. The largerthe lens row pitch Plsr is, the larger the displacements of the spotlatent image formation positions in the width direction LTD tend to be.In contrast, in the above embodiment, the pitches between the lenses LSin the width direction LTD (corresponding to the lens row pitch Plsr)are suppressed and relatively short. Thus, even if the circumferentialspeed should vary, the influence of this circumferential speed variationon the latent image forming operation can be suppressed, which makes itpossible to realize good exposure and latent image forming operation.

In the above embodiment, organic EL devices are used as the lightemitting elements 2951 and these organic EL devices have smaller lightquantities as compared with LEDs (light emitting diodes) and the like.Hence, the light quantities introduced to the lenses LS tend todecrease. Particularly, in the case of using bottom emission-typeorganic EL devices, light beams emitted from the organic EL devices arepartly absorbed by the head substrate 293. Thus, the light quantitiesintroduced to the lenses LS are further decreased. However, since thelenses. LS are shaped to be long in the longitudinal direction LGD(first direction) in this embodiment, larger quantities of lights can beincident on the lenses LS. Therefore, a good exposure is possible evenin a construction using bottom emission-type organic EL devices as thelight emitting elements 2951.

B-2. Second Embodiment

FIG. 13 is a partial sectional view showing a second embodiment of theinvention. In FIG. 13, a construction shown in a large chaindouble-dashed line circle is the enlargement of a construction shown ina small chain double-dashed line circle. As shown in FIG. 13, lensesLS1, LS2 formed on two lens arrays 299A, 299B are convex toward lightemitting element groups 295. In other words, surfaces of the lenses LSfacing the light emitting element groups 295 (light emitting elements2951) are convex surfaces. In this second embodiment, diaphragms DIA areprovided between the lenses LS1 and the light emitting element groups295. These diaphragms DIA are formed by perforating apertures AP in adiaphragm flat plate 298.

The diaphragm DIA and the lens LS (LS1) are in the following positionalrelationship in a light beam propagation direction Doa. Specifically,the diaphragm DIA is arranged in a range within 10% of the sag Lsg ofthe lens LS from a top Lt of the lens LS (top of the convex surface ofthe lens LS) in the light beam propagation direction Doa. This is morespecifically described using the large chain double-dashed line circleof FIG. 13. First of all, when a straight line L(0) is a straight linepassing the top Lt of the lens LS and parallel to the longitudinaldirection LGD, a distance between this straight line L(0) and an undersurface 2991-t of a lens array substrate 2991 in the light beampropagation direction Doa is the sag Lsg of the lens LS. When a straightline at a distance of 0.9×Lsg from the lens array substrate undersurface 2991-t in the light beam propagation direction Doa and parallelto the longitudinal direction LGD is a straight line L(−1) and astraight line at a distance of 1.1×Lsg from the lens array substrateunder surface 2991-t in the light beam propagation direction Doa andparallel to the longitudinal direction LGD is a straight line L(+1), thediaphragm DIA is arranged between the straight lines L(−1) and L(+1) inthe light beam propagation direction Doa. Particularly in the secondembodiment, the diaphragm DIA is located more toward an image plane sidethan the top Lt of the lens LS, that is, the diaphragm DIA is arrangedbetween the straight lines L(0) and L(−1) in the light beam propagationdirection Doa. In other words, a position P1 of the top Lt in thepropagation direction Doa of a light beam originating from the lightemitting element 2951 and a position P2 of the diaphragm DIA in thepropagation direction Doa of the light beam originating from the lightemitting element 2951 satisfy the following formula:P1≦P2≦P1+0.1×Lsg.

FIG. 14 is a partial plan view showing the structure of the diaphragmsaccording to the second embodiment. In FIG. 14, the lenses LS are shownin broken line. This is to show the relationship of the lenses LS1 andthe diaphragms DIA, but does not indicate that the lenses LS1 areprovided on the diaphragm flat plate 298. First of all, the structure ofthe lenses LS1 according to the second embodiment in a plan view isdescribed below. The lenses LS1 have an elliptical shape in the planview. A length L1 (lens main scanning width L1) of each lens LS1 in thelongitudinal direction LGD and a length L2 (lens sub scanning width L2)thereof in the width direction LTD satisfy the following formula:1<L1/L2<1.2.Further, the lenses LS1 are aligned at lens pitches Pls in thelongitudinal direction LGD while being arranged at lens row pitches Plsrin the width direction LTD.

Next, the structure of the diaphragms in a plan view is described. Asshown in FIG. 14, the diaphragm flat plate 298 is provided with aplurality of diaphragms DIA in a one-to-one correspondence with aplurality of lenses LS1 and the geometric centers of the lenses LS1 andthe diaphragms DIA in the correspondence relationship coincide. In thesecond embodiment, a length La1 (diaphragm main scanning diameter La1)of each diaphragm DIA in the longitudinal direction LGD and a length.La2 (diaphragm sub scanning diameter La2) thereof in the width directionLTD satisfy the following formula:1<La1/La2.Particularly in the second embodiment, the following formula:L1/L2=La1/La2is satisfied. Further, the respective diaphragms DIA have an ellipticalshape similar (identical) to the corresponding lenses LS1.

As described above, in the second embodiment, the lens main scanningwidth L1 and the lens sub scanning width L2 satisfy the followingformula:1<L1/L2.In other words, the length of each lens LS in the width direction LTD isset to be shorter, whereas the length thereof in the longitudinaldirection LGD is set to be longer. Accordingly, larger quantities oflights can be incident on the lenses LS in the longitudinal directionLGD while pitches (lens row pitches Plsr) between the lenses LS in thewidth direction LTD are suppressed. Therefore, a line head 29 can beminiaturized while a good exposure at a high resolution is enabled.

Further, in the second embodiment, the lens main scanning width L1 andthe lens sub scanning width L2 satisfy the following relationship:L1/L2<1.2.By employing such a construction, the lenses LS with little astigmatismcan be easily formed by suppressing a difference between the lens mainscanning width L1 and the lens sub scanning width L2, which makes itpossible to easily realize a good exposure. Particularly, in the case offorming lenses using a mold, the construction satisfying the formula ofL1/L2<1.2 is preferable. Specifically, in lens formation using a mold,the lenses are released from the mold by letting the lenses LS contractwith respect to the mold. At this time, if the difference between thelens main scanning width L1 and the lens sub scanning width L2 is large,a degree of contraction of the lens in the longitudinal direction LGD(main scanning direction MD) and the one in the width direction LTD (subscanning direction SD) differ, and hence, astigmatism is likely tooccur. In contrast, by constructing to satisfy the formula of L1/L2<1.2,astigmatism can be easily suppressed to a trouble-free level and a goodexposure can be easily realized.

The second embodiment is preferable since the diaphragm main scanningdiameter La1 and the diaphragm sub scanning diameter La2 satisfy therelationship defined by the following formula:1<La1/La2.In other words, as described above, the lenses LS1 have a property ofreceiving larger quantities of lights in the main scanning direction MD(longitudinal direction LGD) in the second embodiment, whereas thediaphragms DIA are for shielding parts of lights propagating from thelight emitting elements 2951 toward the lenses LS1. Accordingly, inlight of effectively utilizing the lens property of this embodiment, thediaphragms DIA are preferably so shaped as to be advantageous in lettinglarger quantities of lights incident on the lenses in the main scanningdirection MD (longitudinal direction LGD) in order to effectivelyutilize lights from the light emitting elements 2951 by suppressingunnecessary light shielding by the diaphragms DIA. In this respect,since the formula of 1<La1/La2 is satisfied in the second embodiment,larger quantities of lights can be incident on the lenses LS1 in themain scanning direction MD (longitudinal direction LGD), which enables agood exposure.

Further, the second embodiment is constructed such that the lens mainscanning width L1, the lens sub scanning width L2, the diaphragm mainscanning diameter La1 and the diaphragm sub scanning diameter La1satisfy the following formula:L1/L2=La1/La2.Accordingly, lights from the light emitting elements 2951 can be moreeffectively utilized.

Furthermore, the second embodiment is constructed such that the shape ofthe lenses LS1 and that of the diaphragms DIA are similar, which makesit possible to more effectively utilize lights from the light emittingelements 2951.

Further, the second embodiment is constructed such that the diaphragmsDIA are located in the range within 10% of the sags Lsg of the lenses LSfrom the tops Lt of the lenses LS1. Accordingly, lights from the lightemitting elements 2951 can be very effectively utilized by suppressingunnecessary light shielding by the diaphragms DIA. In addition, thediaphragms DIA are located more toward the image plane side than thetops Lt of the lenses LS. Therefore, the utilization efficiency oflights from the light emitting elements 2951 can be more improved.

C. Miscellaneous

As described above, in the above embodiments, the longitudinal directionLGD and the width direction LTD are orthogonal to or substantiallyorthogonal to each other, the main scanning direction MD and the subscanning direction SD are orthogonal to or substantially orthogonal toeach other, the longitudinal direction LGD and the main scanningdirection MD are parallel to or substantially parallel to each other andthe width direction LTD and the sub scanning direction SD are parallelto or substantially parallel to each other. Thus, the longitudinaldirection LGD and the main scanning direction MD correspond to a “firstdirection” of the invention, and the width direction LTD and the subscanning direction SD correspond to a “second direction” of theinvention. The lenses LS are arranged in the first direction and in thesecond direction. Further, the length L1 corresponds to a “lens firstdirection length” of the invention, the length L2 to a “lens seconddirection length” of the invention, the length La1 to a “diaphragm firstdirection length” of the invention and the length La2 to a “diaphragmsecond direction length” of the invention. The lens array substrate 2991corresponds to a “lens substrate” of the invention. Further, the headsubstrate 293 corresponds to a “light emitting element substrate” of theinvention. When the lenses LS1 are “first lenses” of the invention, thelenses LS2 correspond to “second lenses” of the invention. Further, theline head 29 corresponds to an “exposure head” of the invention.Furthermore, the photosensitive drum 21 corresponds to a “latent imagecarrier” of the invention.

The invention is not limited to the above embodiments and variouschanges other than the above can be made without departing from the gistthereof. For example, in the above embodiments, four light emittingelements 2951 are aligned in the longitudinal direction LGD in eachlight emitting element row 2951R, and two light emitting element rows2951R are arranged in the width direction LTD in each light emittingelement group 295. However, the number of the light emitting elements2951 constituting the light emitting element row 2951R and the number ofthe light emitting element rows 2951R constituting the light emittingelement group 295 are not limited to these. Accordingly, the lightemitting element group 295 can be configured as described below.

FIG. 15 is a plan view showing other structure of the light emittingelement groups. FIG. 16 is a view showing the structure of the undersurface of the head substrate on which the plurality of light emittingelement groups shown in FIG. 15 are arranged and corresponds to a casewhere the under surface is viewed from the top surface of the headsubstrate. In the structure shown in FIG. 15, fifteen light emittingelements 2951 are arranged side by side in the longitudinal directionLGD to form the light emitting element rows 2951R. That is, theembodiment shown in FIG. 15 corresponds to the case where m=15. In thelight emitting element rows 29518, the light emitting elements 2951 arearranged at pitches (=0.084 [mm]) which are four times as large as theelement pitches Pel (=0.021 [mm]). Four such light emitting element rows2951R (2951R-1, 2951R-2, 2951R-3, 2951R-4) are arranged in the widthdirection LTD. That is, the embodiment shown in FIG. 15 corresponds tothe case where n=4. In the width direction LTD, the pitch between thelight emitting element row 2951R-4 and the light emitting element row2951R-1 is 0.1155 [mm], the pitch between the light emitting element row2951R-4 and the light emitting element row 2951R-2 is 0.084 [mm], andthe pitch between the light emitting element row 2951R-4 and the lightemitting element row 2951R-3 is 0.0315 [mm]. Further, when a straightline which is parallel to the longitudinal direction LGD and passesthrough the center (of gravity) of the light emitting element group 295is a center line CTL, the pitch in the width direction LTD between thelight emitting element row 2951R-1 and the center line CTL and thatbetween the light emitting element row 2951R-4 and the center line CTLare 0.05775 [mm], respectively.

In FIG. 15, the two light emitting element rows 2951R-1 and 2951R-2above the center line CTL constitute a light emitting element set 2951RTand the two light emitting element rows 2951R-3 and 2951R-4 below thecenter line CTL constitute a light emitting element set 2951RT. In eachlight emitting element set 2951RT, two light emitting element rows 2951Rare shifted from each other in the longitudinal direction LGD by a pitch(=0.042 [mm]) which is twice as large as the element pitch Pel (=0.021[mm]). Further, the two light emitting element sets 2951RT are shiftedfrom each other in the longitudinal direction LGD by the element pitchPel (=0.021 [mm]). Hence, the four light emitting element rows 2951R areshifted from each other in the longitudinal direction LGD by the elementpitches Pel (=0.021 [mm]). As a result, the positions of the lightemitting elements 2951 in the longitudinal direction LGD are different.When the light emitting elements 2951 at the both ends of the lightemitting element groups 295 in the longitudinal direction LGD are calledend light emitting elements 2951 x, the pitch between the end lightemitting elements 2951 x in the longitudinal direction LGD is 1.239 [mm]and the pitch between the end light emitting element 2951 x and thecenter of the light emitting element group 295 in the longitudinaldirection LGD is 0.6195 [mm].

In the embodiment shown in FIG. 16, the light emitting element groups295 shown in FIG. 15 are two-dimensionally arranged. As shown in FIG.16, the plurality of light emitting element groups 295 are arranged inthe longitudinal direction LGD to form the light emitting element grouprows 295R. In the light emitting element group rows 295R, the lightemitting element groups 295 are arranged at pitches (=1.778 [mm]) whichare triple as large as the light emitting element group pitches Peg.Three light emitting element group rows 295R (295R-1, 295R-2, 295R-3)structured in this way are arranged in the width direction LTD at thelight emitting element group row pitches Pegr (=1.77 [mm]). The lightemitting element group rows 295R are shifted from each other in thelongitudinal direction LGD by the light emitting element group pitchesPeg (which are about 0.593 [mm]). That is, the light emitting elementgroup row 295R-1 and the light emitting element group row 295R-2 areshifted from each other in the longitudinal direction LGD by 0.59275[mm], the light emitting element group row 295R-2 and the light emittingelement group row 295R-3 are shifted from each other in the longitudinaldirection LGD by 0.5925 [mm], and the light emitting element group row295R-3 and the light emitting element group row 295R-1 are shifted fromeach other in the longitudinal direction LGD by 0.59275 [mm]. Hence, thelight emitting element group row 295R-1 and the light emitting elementgroup row 295R-3 are shifted from each other in the longitudinaldirection LGD by 1.18525 [mm].

Further, in the above embodiments, the shape of each lens LS is asubstantially reversed fan shape, a substantially fan shape or a flatshape. In other words, the lenses LS of the above embodiments are shapedas if by cutting end(s) of substantially circular lenses. For example,the lenses LS-u are shaped by cutting the lower ends of thesubstantially circular lenses in the longitudinal direction LGD, and thelenses LS-m are shaped by cutting both upper and lower ends of thesubstantially circular lenses in the longitudinal direction LGD.However, the shapes of the lenses LS are not limited to these. In short,the effects of the invention can be exhibited when the lens longitudinaldirection length L1 is longer than the lens width direction length L2.Accordingly, the lenses can be formed, for example, as follows.

FIG. 17 is a diagram showing the construction of a lens array accordingto another embodiment of the invention, and corresponds to a case wherethe lens array is seen from an image plane side (in a light beampropagation direction Doa). Lenses LS in FIG. 17 are formed on an undersurface 2991-t of a lens array substrate 2991, and FIG. 17 shows theconstruction on this lens array substrate under surface 2991-t. Pointsof difference between the above embodiments and this embodiment aremainly described below and common parts are not described.

As shown in FIG. 17, the respective lenses LS have an elliptical shapelong in the longitudinal direction LGD. Accordingly, a lens longitudinaldirection length L1 and a lens width direction length L2 satisfy thefollowing formula:L1>L2.Thus, in this embodiment as well, larger quantities of lights can beincident on the lenses LS in the longitudinal direction LGD whilepitches (corresponding to the lens row pitch Plsr) between the lenses LSin the width direction LTD are suppressed. Therefore, in thisembodiment, a line head 29 can be miniaturized while a good exposure ata high resolution is enabled.

In the above embodiments, three lens rows LSR are arranged in the widthdirection LTD. However, the number of the lens rows LSR is not limitedto three and the invention is applicable to constructions with two ormore lens rows LSR.

Further, in the embodiments above, the lenses LS are formed on the undersurface 2991-t of the lens array substrate to constitute the lens array299. However, the structure of the lens array is not limited to this.That is, the lenses LS may be formed on the top surface 2991-h of thelens array substrate 2991 to constitute the lens array 299, oralternatively, the lenses LS may be formed on the both surfaces 2991-tand 2991-h of the lens array substrate to constitute the lens array 299.

Further, although the two lens arrays 299 are used in the aboveembodiments, the number of the lens arrays 299 is not limited to this.

In the above embodiments, organic EL devices are used as the lightemitting elements 2951. However, the devices other than the organic ELdevices may be used as the light emitting elements 2951. For example,LEDs (light emitting diodes) may be used as the light emitting elements2951.

D. Examples

Next, examples of the invention are described, but the invention is notrestricted by the following examples and can be, of course, embodied bybeing appropriately changed within the scope conformable to the gistdescribed above and below. Any of these examples are embraced by thetechnical scope of the invention.

Examples to be described below have constructions advantageous torealize a good exposure while miniaturizing the image forming apparatus.Specifically, the diameter of the photosensitive drum 21 is an essentialpoint upon determining the size of the image forming apparatus.Accordingly, for the miniaturization of the image forming apparatus, itis desired to make the diameter of the photosensitive drum 21 smaller.However, in addition to the line head 29, functioning units such as thecharger 23 and the developer 25 need to be arranged in the sub scanningdirection SD around the photosensitive drum 21. Thus, there were caseswhere these functioning units could not be arranged if the diameter ofthe photosensitive drum 21 was simply made smaller. In contrast, theline head 29 of the invention is miniaturized in the width direction LTD(sub scanning direction SD). Therefore, the diameter of thephotosensitive drum 21 can be made smaller while a space for arrangingthe respective functioning units is ensured.

However, another problem as described next occurred in some cases if thediameter of the photosensitive drum 21 is made smaller in this way.Specifically, if the diameter of the photosensitive drum 21 is madesmaller, the curvature of the surface shape of the photosensitive drum21 increases. Accordingly, in the case where a plurality of lenses LSare arranged in the width direction LTD as in the line head 29 describedabove, imaged positions by some lenses LS may be displaced from thesurface of the photosensitive drum 21 if the imaging positions in thelight propagation direction Doa are similarly set for the respectivelenses LS. As a result, no good exposure could be performed in somecases. Accordingly, technology enabling the realization of a goodexposure while making the diameter of the photosensitive drum 21 smalleris described in the following examples.

FIG. 18 is a view showing an optical system according to the example andshowing a cross section taken along the main scanning direction MD. Inthis example, a diaphragm DIA is provided in front of the first lens LS1in the light beam propagation direction Doa so that a light beamrestricted by the diaphragm DIA impinges upon the first lens LS1. FIG.18 shows the optical path of a light beam which leaves an object pointOB0, which is on the optical axis OA, and converges at an image pointIM0 and the optical path of a light beam which leaves an object pointOB1, which is different from the optical axis OA, and converges at animage point IM1. The structure other than the diaphragm DIA isapproximately similar to those according to the first embodiment and thelike. The optical systems including the lenses LS are arranged such thatthe three lenses LS-u, LS-m and LS-d are arranged in the direction ofthe line A-A shown in FIG. 5 and the like to form the lens rows.

FIG. 19 is a partial cross sectional view of the line head and thephotosensitive drum taken along the line A-A according to the example.As shown in FIG. 19, the line head formed by the light emitting elementgroups 295, the diaphragm DIA and the lens arrays 299A and 299B isarranged opposed against the photosensitive drum 21. The photosensitivedrum 21 has an approximately cylindrical shape around a rotation axisCC21, and the surface of the photosensitive drum has a finite curvature.The shape of the surface of the photosensitive drum will now bespecifically referred to as a “curvature shape”.

In this example, the respective optical systems are arranged at equalpitches in a horizontal direction in FIG. 19, and the optical axis OA ofthe optical system including the middle lenses LS-m passes through therotation axis CC21 of the photosensitive drum 21. Hence, in order toapproximately coincide the image forming positions at which the opticalsystems focus light beams with the surface of the photosensitive drum,it is necessary to adjust, for each optical system, the image formingposition in the light beam propagation direction Doa (that is, thedirection of the optical axes OA). In the example shown in FIG. 19,between the optical systems which include the upstream lenses LS-u andthe optical systems which include the downstream lenses LS-d, the imageforming positions FP in the light beam propagation direction Doa areequal to each other. On the other hand, between the optical systemswhich include the upstream lenses LS-u (or the downstream lenses LS-d)and the optical systems which include the middle lenses LS-m, the imageforming positions in the light beam propagation direction Doa aredifferent from each other by a distance ΔFP. As the data below show, inthis example, the optical systems which include the lenses LS-u and LS-dhave different structures from the optical systems which include themiddle lenses LS-m.

FIG. 20 is a table showing optical data according to this example. Asshown in FIG. 20, the wavelength of light beams emitted from the lightemitting elements is 690 [nm]. The diameter of the photosensitive memberis 40 [mm]. FIG. 21 is a table showing the data of the optical systemswhich include the middle lenses. As shown in FIG. 21, in the opticalsystems which include the middle lenses LS-m, the lens surfaces (denotedat the surface number S4) of the first lenses LS1 and the lens surfaces(denoted at the surface number S7) of the second lenses LS2 are bothfree-form surfaces (X-Y polynomial surfaces). FIG. 22 is a drawing ofdefinitional equations which define the X-Y polynomial surfaces. Theshape of the lens surfaces of the first lenses LS1 is expressed by theseequations and the coefficients which are shown in FIG. 23. The shape ofthe lens surfaces of the second lenses LS2 is expressed by theseequations and the coefficients which are shown in FIG. 24. FIG. 23 is atable of the coefficients indicative of the surfaces S4 of the opticalsystems which include the middle lenses, and FIG. 24 is a table of thecoefficients indicative of the surfaces S7 of the optical systems whichinclude the middle lenses.

FIG. 25 is a table showing data of an optical system including upstreamlenses and downstream lenses. As shown in FIG. 25, also in the opticalsystem including the upstream lenses LS-u and downstream lenses LS-d,both lens surfaces (surface number S4) of the first lenses LS1 and those(surface number S7) of the second lenses LS2 are free-form surfaces (XYpolynomial surfaces). The lens surface shape of the first lenses LS1 isgiven by the definition formula of FIG. 22 and a coefficient shown inFIG. 26 and that of the second lenses LS2 is given by the samedefinition formula and a coefficient shown in FIG. 27. Here, FIG. 26 isa table showing coefficient values of the surfaces S4 of the opticalsystem including the upstream and downstream lenses, and FIG. 27 is atable showing coefficient values of the surfaces S7 of the opticalsystem including the upstream and downstream lenses.

By applying the invention also to such an optical system so that thelens longitudinal direction length L1 and the lens width directionlength L2 satisfy the following formula:L1>L2,larger quantities of lights can be incident on the lenses LS in thelongitudinal direction LGD while pitches (corresponding to the lens rowpitch Plsr) between the lenses LS in the width direction LTD aresuppressed.

Further, the imaging positions of the respective lenses LS are adjustedin conformity with the surface shape of the photosensitive drum 21.Accordingly, a good exposure can be realized while the miniaturizationof the image forming apparatus is promoted by making the diameter of thephotosensitive drum 21 smaller.

In the above example, the lenses LS of the lens array 299 are free-formsurface lenses. Accordingly, the imaging characteristic of the lenses isimproved and a better exposure can be realized. Here, the free-formsurface lenses are lenses whose lens surfaces are free-form surfaces.

The diameter of the photosensitive drum 21 is not limited to the abovevalue and can be changed. Accordingly, the diameter of thephotosensitive drum 21 may be changed, for example, as shown in FIG. 28to be described next. FIG. 28 is a table showing another numericalexample and corresponds to a case where the diameter of thephotosensitive drum 21 is 30 [mm]. As shown in FIG. 28, the lenslongitudinal direction length L1 (lens main scanning width L1) is 1.7[mm], whereas the lens width direction length L2 (lens sub scanningwidth L2) is suppressed to 1.5 [mm]. As a result, the lens row pitchPlsr is suppressed to 1.54 [mm]. Further, the lens pitch Pls is 1.778[mm].

In order to adjust imaging positions FP for each lens row LSR inconformity with the shape of the photosensitive drum 21 having adiameter of 30 [mm], the imaging positions are changed in the opticalsystem including the upstream lenses LS-u (or downstream lenses LS-d)and the optical system including the middle lenses LS-m. Specifically, adistance ΔFP=0.78 [mm]. The distance ΔFP in this numerical example isobtained based on the data of the optical systems shown in FIGS. 19 to27.

FIG. 29 is a table showing still another numerical example andcorresponds to a case where the diameter of the photosensitive drum 21is 45 [mm]. In this numerical example, the lens row pitch Plsr issuppressed to 1.5 [mm]. Further, in order to adjust the imagingpositions FP for each lens row LSR in conformity with the shape of thephotosensitive drum 21 having a diameter of 45 [mm], the imagingpositions are changed in the optical system including the upstreamlenses LS-u (or downstream lenses LS-d) and the optical system includingthe middle lenses LS-m. Specifically, a distance ΔFP=0.05 [mm].

As described above, in this still another numerical example shown inFIG. 29, the distance ΔFP is suppressed to a smaller value as comparedwith the above another numeral example shown in FIG. 28. As a result, alens design can be simplified without needing to drastically change thelens characteristic for each lens LS. This is caused by setting the lensrow pitch Plsr (=1.5 [mm]) shorter with respect to the diameter (=45[mm]) of the photosensitive drum 21. Upon simplifying the lens design inthis way, the lens row pitch Plsr may be set to or below 1/20 of thediameter (=45 [mm]) of the photosensitive drum 21.

Further, in an embodiment of an aspect of the invention, a length L1 ofthe lens in the first direction and a length L2 thereof in the seconddirection may satisfy a relationship defined by a following formula:L1/L2<1.2. By employing such a construction, lenses with littleastigmatism can be easily formed by suppressing the difference betweenthe length L1 of the lens in the first direction and the length L2thereof in the second direction. Hence, a good exposure can be easilyrealized.

Further, a lens array may include a lens substrate on which the lensesare formed. By constructing the lens array with the lens substrate andthe lenses in this way, a degree of freedom in the construction of thelens array is improved, for example, by enabling the selection ofdifferent base materials for the lens substrate and the lenses. Thus,the lens array can be appropriately designed depending on specificationrequired for the exposure head, which enables to more easily realize agood exposure by the exposure head.

An aspect of the invention is also applicable to a constructionincluding a diaphragm arranged between the light emitting element andthe lens. As described above, according to an aspect of the invention,the lenses have a property of receiving large quantities of light in thefirst direction, whereas the diaphragm is designed to shield part oflight propagating from the light emitting element toward the lens.Accordingly, in light of effectively utilizing the lens property of theinvention, the diaphragm is preferably so shaped as to be advantageousin letting large quantities of light incident on the lens in the firstdirection in order to effectively utilize light from the light emittingelement by suppressing unnecessary light shielding by the diaphragm.Thus, the length La1 of the diaphragm in the first direction (diaphragmfirst direction length La1) and the length La2 thereof in the seconddirection (diaphragm second direction length La2) may satisfy arelationship defined by the following formula: 1<La1/La2. In this way,larger quantities of light can be incident on the lens in the firstdirection and a good exposure is possible.

At this time, the lens first direction length L1, the lens seconddirection length L2, the diaphragm first direction length La1 and thediaphragm second direction length La2 may satisfy the following formula:L1/L2=la1/La2. In this way, light from the light emitting element can bemore effectively utilized.

Further, the shape of the lens and that of the diaphragm may beidentical. In this way, light from the light emitting element can beeven more effectively utilized.

The diaphragm may have elliptical shape.

Further, the surface of the lens facing the light emitting element maybe a convex surface. At this time, the utilization efficiency of lightfrom the light emitting element can be further improved by arranging thediaphragm more toward the image plane side than the top of the convexsurface of the lens.

Further, the lenses may be free-form surface lenses. This is because theimaging characteristic of the lenses is improved and a better exposurecan be realized by employing the free-form surface lenses.

An embodiment of a line head according to another aspect of theinvention comprises a head substrate and a lens array. Light emittingelement groups each formed by grouping light emitting elements arearranged on the head substrate. The lens array includes a lens arraysubstrate having lenses arranged thereon in a one-to-one correspondencewith the light emitting element groups. Lights from the light emittingelement groups are incident on the lenses corresponding to the lightemitting element groups. Lens rows each made up of lenses aligned in afirst direction are arranged in a second direction orthogonal to orsubstantially orthogonal to the first direction on the lens arraysubstrate. The following formula: L1>L2 is satisfied, where the symbolL1 denotes a length of the lens in the first direction and the symbol L2denotes a length thereof in the second direction.

An embodiment of an image forming apparatus according to still anotheraspect of the invention comprises a line head and a latent image carrierthat is exposed by the line head. The line head includes a headsubstrate and a lens array. Light emitting element groups each formed bygrouping light emitting elements are arranged on the head substrate. Thelens array includes a lens array substrate having lenses arrangedthereon in a one-to-one correspondence with the light emitting elementgroups. Lights from the light emitting element groups are incident onthe lenses corresponding to the light emitting element groups. Lens rowseach made up of lenses aligned in a first direction are arranged in asecond direction orthogonal to or substantially orthogonal to the firstdirection on the lens array substrate. The following formula: L1>L2 issatisfied, where the symbol L1 denotes a length of the lens in the firstdirection and the symbol L2 denotes a length thereof in the seconddirection.

In each of the embodiments (line head, image forming apparatus) thusconstructed, the lens array substrate having the lenses arranged thereonin one-to-one correspondence with the light emitting element groups isprovided and the lens rows each made up of the lenses aligned in thefirst direction are arranged in the second direction orthogonal to orsubstantially orthogonal to the first direction on this lens arraysubstrate. When L1 denotes the length of the lens in the first directionand L2 denotes the length thereof in the second direction, the line headis so constructed as to satisfy the following formula: L1>L2. In otherwords, the length of the lens in the second direction is set to beshorter, whereas the length thereof in the first direction is set to belonger. Accordingly, larger quantities of lights can be incident on thelenses in the first direction while pitches between the lenses in thesecond direction are suppressed. Therefore, in each of the embodiments,the line head can be miniaturized while a good exposure at a highresolution is enabled.

Further, in each of the embodiments, the lens array includes the lensarray substrate and the lenses are arranged on the lens array substrate.Accordingly, a degree of freedom in the construction of the lens arrayis improved, for example, by enabling the selection of different basematerials for the lens array substrate and the lenses. Thus, the lensarray can be appropriately designed depending on specification requiredfor the line head and a good exposure by the line head can be easilyrealized.

The lens array substrate may be made of a glass. In other words, theglass has a relatively small linear expansion coefficient. Accordingly,the deformation of the lens array caused by a temperature change can besuppressed by making the lens array substrate of the glass, and hence, agood exposure can be realized independently of temperature.

The lenses may be made of a light curing resin. In other words, thelight curing resin is cured upon light irradiation. Accordingly, thelens array can be easily produced by making the lenses of this lightcuring resin, and hence, the cost of the line head can be suppressed.

The embodiment is particularly preferably applied to a line headincluding organic EL devices as the light emitting elements. In otherwords, in the case where the organic EL devices are used as the lightemitting elements, light quantities of the light emitting elements aresmaller as compared with the case using LEDs or the like. This isparticularly notable in the case of using bottom emission-type organicEL devices as the light emitting elements. Accordingly, in order torealize a good exposure, it is preferable to let larger quantities oflights incident on the lenses by applying the embodiment.

The embodiment is particularly preferably applicable to an image formingapparatus in which a surface of the latent image carrier is moved in thesecond direction and light emitting elements of a line head are drivenfor light emission at timings in conformity with a movement of thelatent image carrier surface to expose the latent image carrier surface.In other words, as described earlier, there were cases where an increasein pitches between the lenses in the second direction caused an exposurefailure in such an image forming apparatus. In contrast, in the case ofapplying the embodiment, the pitches between the lenses in the seconddirection are suppressed and a good exposure is possible.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asother embodiments of the present invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that the appended claims willcover any such modifications or embodiments as fall within the truescope of the invention.

1. An image forming apparatus, comprising: an exposure head thatincludes a light emitting element substrate that is provided with lightemitting elements that emit lights and are arranged in a first directionand a second direction orthogonal or substantially orthogonal to thefirst direction, a lens array that has lenses that are arranged in thefirst direction and the second direction and image a light emitted fromthe light emitting elements, and a diaphragm that is arranged betweenthe light emitting elements and the lenses; and a latent image carrierthat is exposed by the exposure head to form a latent image and moves inthe second direction, wherein a relationship defined by a followingformula:1<L1/L2 is satisfied, where the symbol L1 denotes a length of the lensin the first direction and the symbol L2 denotes a length of the lens inthe second direction.
 2. The image forming apparatus according to claim1, wherein a relationship defined by a following formula:L1/L2<1.2 is satisfied.
 3. The image forming apparatus according toclaim 1, wherein the lens array includes a lens substrate on which thelenses are formed.
 4. The image forming apparatus according to claim 1,wherein a relationship defined by a following formula:1<La1/La2 is satisfied, where the symbol La1 denotes a length of thediaphragm in the first direction and the symbol La2 denotes a length ofthe diaphragm in the second direction.